Please refer to FIG. 7-7D, which are the schematics of conventional production processes of acoustic wave device. First, forming a recess 702 on a silicon substrate 701; then forming a protection layer 703 on the silicon substrate 701 and the recess 702; and then forming a phosphosilicate glass (PSG) layer 705 on the protection layer 703 such that the phosphosilicate glass (PSG) layer 705 at least filled the recess 702; then polishing to remove the phosphosilicate glass (PSG) layer 705 outside the recess 702 by chemical mechanical polishing (CMP). Forming an acoustic wave device 710 with metal 711—insulator 712—metal 713 structure above the recess 702 such that the two ends of the acoustic wave device 710 with metal 711—insulator 712—metal 713 structure across outside of the recess 702; removing the rest of the phosphosilicate glass (PSG) layer 705 within the recess 702 such that the recess 702 forms a cavity.
Conventional technical producing the acoustic wave device needs to apply chemical mechanical polishing (CMP) technique for polishing to remove the phosphosilicate glass (PSG) layer 705 outside the recess 702. Furthermore the polishing requires fine polishing such that the roughness of polished surface is very smooth. Otherwise, the formation of the acoustic wave device 710 with metal 711—insulator 712—metal 713 structure will be influenced by the roughness of the polished surface. However the fine polished surface requirement for chemical mechanical polishing (CMP) process, not only the cost of the equipment is very expensive but also the time consuming and the materials cost are very high, such that the cost of production is too high.
Furthermore, the design of the single recess 702 has the problem that the gap between the bottom of the acoustic wave device 710 and the bottom of the recess 702 cannot efficiently widen. Hence, when the acoustic wave device 710 is affected by stress such that the acoustic wave device 710 is bended downwardly, the bottom of the acoustic wave device 710 may easily contact with the bottom of the recess 702 such that the characteristics of the acoustic wave device 710 been affected.
On the other hand, the application of the acoustic wave device 710 is often used as a radio frequency signal filter. When the application is with the power amplifier, the acoustic wave device plays a role to filter the signal firstly and then transmits the filtered signal to the power amplifier; or the power amplifier amplifies the signal firstly and then transmits the amplified signal to the acoustic wave device for filtering. However, the conventional acoustic wave device design is usually based on the silicon substrate. There is no one who ever tries to integrate the acoustic wave device with the compound semiconductor power amplifier on the same compound semiconductor epitaxial substrate. Integrating the acoustic wave device and the power amplifier on the same compound semiconductor epitaxial substrate may reduce the component size, and optimize the impedance matching, and reduce the signal loss between the power amplifier and the acoustic wave device.
Accordingly, the inventor has developed the design which may effectively widen the gap between the bottom of the acoustic wave device and the bottom of the recess, also may integrate the acoustic wave device and the power amplifier on the same compound semiconductor epitaxial substrate with the above mentioned benefits, the advantage of low cost, and with reduced component size, the optimized impedance matching, and the reduced signal loss between the power amplifier and the acoustic wave device.